Frequency control system for oscillators



Oct. 15, 1957 A. HAHNEL 2,810,074

FREQUENCY coNTRII SYSTEM FOR OSCILLATORS Filed June 2, 1955 AMPLITUDE FUNCTION AMP =nf I TIME 0 FIG I FIG.2

AMPL'TUDE SELECTIVE PPGO MOD DETECTOR AMPLIFIER AMP l2 l4 m REAOTANCE 2 PHASE 4- v L AGE MOD SOURCE COMPARATOR (n |)fl 0 mm ("Hm A FE FIG. 4

FIG.3

I 5 A c AMP B I I TIME I FIG. 6 I 5 C I I I 5 D i INVENTOR.

| ALWIN HAHNEL W 7 jaw ATTORNEY United States FREQUENCY CONTROL SYSTEM FOR OSCILLATORS Alwin Hahnel, Little Silver, N. J., assignor to the United States of America as represented by the Secretary of the Army Application June 2, 1955, Serial No. 512,910

7 6 Claims. (Cl. 250-36) (Granted under Title 35, U. S. Code (1952), sec. 26a) output frequencies in a limited band of the frequency range is based on the fact that the higher-frequency oscillations are keyed and phase controlled such that the output waveform is periodic at the fundamental frequency f,. This type of circuit is defined as a periodically phase-con trolled oscillator. In utilizing such a circuit as an oscillator, it has been found that the unwanted adjacent harmonies were suppressed well below the desired output frequency. Because of this unique characteristic it is desirable to utilize such an oscillator as a transmitter exciter or, in receivers, as a beat frequency oscillator. In either case it is required that the operating frequency be maintained substantially constant to obtain a maximum supression of undesired harmonics. The term operating frequency refers to the frequency of the resonant circuit of the oscillator and is substantially equal to the desired out put harmonic or multiple frequency f,=nf,. As a beat frequency oscillator, it is especially desirable that the operating frequency of the periodically phase-controlled oscillator be maintained such that there is'a maximum ratio between the desired frequency output and any unwanted harmonics so that the desired signal will be at all times differentiated from any unwanted spurious responses. However, it has been found that when the output energy of the periodically phase-controlled oscillator is concentrated at one frequency, there is an increased tendency for the operating frequency of such an oscillator to drift which causes a shift in the spectrum envelope and, in effect, decreases the amplitude of the desired harmonic frequency and increases the amplitude of the undesired harmonic frequencies.

It is therefore an object of the present inventionto provide a control system wherein the operating frequency of the periodically phase-controlled oscillator is automati cally maintained such that there is provided a maximum ratio between the desired harmonic frequency output from the oscillator and any unwanted adjacent harmonics.

It is another object of the present invention to provide a control system wherein the maximum amplitude of a desired harmonic generated in a periodically phase-controlled oscillator is automatically maintained.

It is still another object of the present invention to provide an automatic frequency control system wherein there is obtained a high order of suppression of the unwanted adjacent harmonics during the operation of a periodically phase-controlled oscillator that is characteratent Q ized by a spectrum amplitude envelope similar to that of a single frequency source. i

In accordance with the present invention there is provided a system for maintaining a prescribed harmonic frequency output from a periodically phase-controlled oscillator adapted to produce a relatively narrow frequency spectrum envelope such that the ratio between the spectrum amplitude at the prescribed harmonic frequency and adjacent harmonics is a maximum. The periodically phase-controlled oscillator operates in the mode where its output wave form (operating frequency) has a maximum amplitude duration and where the area under the wave form envelope varies slightly due to th'e variation in the amplitude function of the output wave form. Included is a source of modulating voltage and means for varying the amplitude function of the operating frequency of the oscillator in accordance with the modulating voltage. Also included are means for detecting the variations in the amplitude function whereby a signal voltage is produced either out of phase or in phase with the modulating voltage when the operating frequency of the oscillator differs from the prescribed harmonic frequency.

When the oscillator is operating at the desired frequency,

the oscillator is tuned in a direction to decrease the difference between the operating frequency and the desired prescribed frequency.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which:

Figure 1 illustrates the output wave shape from the periodically phase-controlled oscillator;

Figure 2 illustrates a typical output frequency spectrum;

Figure 3 is a block diagram of the present invention, and

Figures 4-6 are explanatory diagrams.

Figure 1 illustrates the operating frequency wave form output with respect to time from the periodically phasecontrolled oscillator described in the aforementioned application. Such a wave form can be divided into an exponential build-up period T1, a degenerative period T2, and a constant amplitude period T3 therebetween. The amplitude envelope of the output wave form is termed the amplitude function. For the purpose of this invention, it is to be assumed that the periodically phase-controlled oscillator operates in a mode such that its output wave form has a constant amplitude section of maximum obtainable duration, and hence relatively small build-up and decay periods, when operating at the desired frequency. With such a'tirne function, the output energy of the periodically phase-controlled oscillator is concentrated at one frequency as illustrated by the spectrum envelope shown in Figure 2 having maximum amplitude at the prescribed multiple frequency f =n,f If such an oscillator is detuned, the spectrum envelope is shifted, for example, to

the right for an increase in frequency or tothe left for :a'

decrease in frequency. This detuning manifests itself as a variation in the amplitude function which in turn affects the area under the output wave form shown in Figure 1.

The control system in accordance with the present in- Patented Oct. 15, 1957 vention is illustrated in Figure}. Referring now to this figure there is shown at 10 a periodically phase-controlled oscillator having the operating characteristics hereinabove described. :Connected tothecircuit determining the ;operating frequency of oscillator 10'is a .conventional reactance tube modulator -12 to whichisapplieda suitable sinusoidalgmodulating voltage from modulating voltage source 14. The frequencyofthemodulating voltage'is such that it does not-interfere inany way with the operating frequencies of the associated equipment. The output of'the oscillator ltl-is-applied to a conventional amplitude modulationdetector 16 whichdemodulates the output from oscillator 10-so1thatthe modulationenvelope is recovered. Theoutput of detector 16-is applied as one inputsignal to a phase comparator circuit 18 through a selective amplifier 19 which is responsive only to the modulating voltage frequency. The modulating voltage from source 14 is applied as the other input signal to phase comparator circuit-18 :which is adapted to produce a voltage of one polarity when the-inputs are in phase, and a voltage-of opposite polarity when the inputs are 180 out of phase. The output of phase comparator 18 is the bias appliedtomodulator 12 which in turn controls the operating frequency of oscillator 10. Inasmuch as blocks 1219 represent conventional circuits well known in the art, no further detailed description thereof is believed necessary.

To betterunderstand the operation of the invention, reference is made to Figures 4-6. Let it be assumed that the desired operating frequency of oscillator 10 is to be maintained substantially constant at f =n,f, as shown-by the solid line spectrum envelope in Figure 4, and that initially there is no output from phase comparator 18 so that the periodically phase-controlled oscillator 10 is operating at f =n,f,. The modulating voltage from source 16 is assumed tobe sinusoidal (Figure A) and as hereinabove stated, has a frequency such that it does not interfere in any way with the operating frequencies of the associated equipment. In addition, the modulating voltage is of a relatively small amplitude so that the undesirable adjacent harmonics are not unnecessarily increased in amplitude. .Under these conditions, the modulating signal applied to reactance modulator 12 will detune the output of oscillator on either side ofthe desired frequency f,,=n,f,. Thus for the positive half cycle of the modulation voltage applied to reactance modulator 12, the output spectrum envelope from oscillator 10 will be shifted to the right, i. e., the resonance frequency is increased as shown by thedotted spectrum envelope B in Figure 4. Similarly, for the negative half cycle of the modulation voltage, the resonance frequency output from oscillator 10' is decreased and the spectrum envelope is shifted to the leftas shown at B. The effect of detuning oscillator 10 in this manner insofar as the operating frequency wave form output from the resonant circuit thereof is concerned is illustrated in Figure 6. The periodic tuning of oscillator 10 will cause the area under the output waveform envelope to gradually vary between two limits C and C, with maximum area at frequency f,,=n,f and minimum area at maximum frequency deviation. Thus foreach half cycle of the modulating voltage, the .constant amplitude duration of the output wave form from oscillator 10 gradually decreases and returns to its original duration. In other words, when maximum detuning takesplace, the duration of the constant amplitude periodis at a minimum andiat the desired frequency f=n,f,, the duration of the constant amplitude period is at a maximum. The output wave form from oscillator 10 is applied to conventional. amplitude diode detector 16-where the variation of the amplitude function is. recovered.

From the discussion hereinabove, it isapparent that with the oscillator tuned correctly at f =n,f, the amplitude function .of the desired harmonic. outputofoscillator 10 will be first decreased and then returned to maximum by is applied to bias reactance modulator 12. Now let it be assumed that the output of oscillatorli) has drifted so that it is now resonant ata-higher frequency than the desired harmonic frequency ,,=n,] (curve B of Figure 4). For this condition, the positive half cycle of the modulation voltage will shift the output spectrum to the right thereby decreasing the areaof'the wave form output of oscillator 10, and the negative half cycle will shiftthe output spectrum to the left thereby increasing this area. Hence the voltage from detector 16 will be that illustrated in Figure 5C. Asshown, the detected voltage is at the same frequency as-the modulation voltage but out of phase therewith. This voltage is now passedthrough selective amplifier 19 to phase comparator 18 which produces a voltage output having a polarity such that reactance modulator 12 isbiased in a direction to reduce the frequency error and hence tune oscillator 10 toward the desiredfrequency.

Ifthe output of oscillator 10 had drifted so'that it was resonant ata frequency lower than the desired frequency f,,=n,f (curve B, Figure 4), then the operation would be reversed. For the positive half cycle of the modulation voltagethe area-of the wave form output from oscillator 10 will be increased and for the negative half cycle of the modulation voltage, the area of the wave form output from oscillator 10 will be decreased. The voltage output from detector 16 isnow in phase with the modulation voltage and. at the same frequency, as shown in Figure'SD. Withthe two inputs to phase comparator 18 now in phase, the corrected output voltage from the phase comparator will be of opposite polarity and is again applied to the reactancemodulator 12 to reduce the frequency errorand tune oscilator 1i toward the desired frequency.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein without departing from the invention, and it is, therefore, airnedinthe appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A system for maintaining a prescribed harmonic frequency output from a periodically phase-controlled oscillator operating substantially at said harmonic frequency and adapted to produce a narrow frequency spectrum envelope such that the ratio betweenthe spectrum amplitude of said prescribed frequency and adjacent harmonies is a maximum comprising a source of modulating voltage, means for varying the amplitude function of said operating frequency in accordance with said modulating voltage, means responsive only to the variations in said amplitude function for detecting the variations whereby a signal voltageis produced either 180 out of phase or in phase with said modulating voltage when the frequency output of said oscillator deviates from said prescribed frequency, means for comparing the phase of the detected voltage and said modulating voltage whereby a correction voltage is produced having a given polarity when said compared voltages are in phase and an opposite polarity when said compared voltages are 180 out of phase, said amplitude function varying means being responsive to said. correction voltage for tuning said oscillator in a direction to decrease the difference between said operating frequency and ,said prescribed frequency.

.2. The. system in. accordance with. claim 1 wherein said modulating voltageis sinusoidal.

3. The system in accordance with claim 1 wherein said varying means is a reactance modulator.

4. The system in accordance with claim 1 wherein the detected voltage from said detecting means is 180 out of phase with said modulating voltage when said operating frequency is higher than said prescribed frequency.

5. In combination, a periodically phase-controlled oscillator adapted to generate a relatively narrow frequency spectrum wherein the ratio between the amplitude of a prescribed harmonic frequency output and adjacent harmonic frequencies is a maximum, a source of modulating voltage, means for varying the amplitude function of the operating frequency of said oscillator in accordance with said modulating voltage, an amplitude modulation detector responsive only to said variations whereby there is produced a detected voltage envelope having twice the frequency of said modulation voltage when the output energy of said oscillator is concentrated at said prescribed frequency, a detected voltage envelope in phase with said modulating voltage when the operating frequency of said oscillator is less than said prescribed frequency, or a detected voltage envelope 180 out of phase with said modulating voltage when the operating frequency of said oscillator is greater than said prescribed frequency, amplifying means responsive only to the frequency of said modulation source, a phase comparator responsive to both the output of said amplifying means and said modulating voltage whereby there is produced a correction voltage having a given polarity when the compared voltages are in phase and an opposite polarity when the compared voltages are 180 out of phase, said varying means being responsive to said correction voltage for tuning said oscillator in a direction to decrease the difference between the operating frequency and said prescribed frequency.

6 frequency output from a periodically phase-controlled oscillator operating substantially at said harmonic frequency and adapted to produce a narrow frequency spectrum envelope such that the ratio between the spectrum amplitude of the prescribed frequency and adjacent harmonics is a maximum comprising a source of modulating voltage, means for varying the amplitude function of said operating frequency in accordance with said modulating voltage, means responsive only to the variations in said amplitude function for generating a signal voltage having a frequency twice that of said modulating voltage when said operating frequency is equal to said prescribed frequency, and a signal voltage either in phase or 180 out of phase with said modulating voltage when the operating frequency deviates from said prescribed frequency, means responsive to said signal voltage and said modulating voltage for producing a correction voltage of one polarity when said detected signal is 180 out of phase with said modulating voltage and a correction voltage of opposite polarity when said detected signal is in phase with said modulating voltage, said amplitude function varying means being responsive to said correction voltage for tuning said oscillator in a direction to decrease the difference between the output frequency and the prescribed frequency.

References Cited in the file of this patent UNITED STATES PATENTS Halpern et al Aug. 14, 1951 OTHER REFERENCES 6. A system for maintaining a prescribed harmonic 

